Hearing aid technology enjoys a long and colorful history. Early hearing aids used in the 18th and 19th centuries were often referred to as ear trumpets. They essentially consisted of a large horn, or bell, that tapered into a thinner tube for placement in or near the ear. They were large, bulky passive devices that simply increased the volume of sound and provided some noise filtering by directing the desired sound directly into the ear.
Around the turn of the 20th century, electronic hearing aids began to enter the market. These were tabletop or desktop items that were cumbersome and impractical, but they provided electronic amplification of the desired sound. While desktop devices were reduced in size over the next few decades, they were still cumbersome units and their battery life was typically only a few hours. With reduction in the size of vacuum tubes, hearing aids shrunk to the point that they were considered “pocket-sized” or “wearable,” but were still bulky and required large batteries.
With the advent of the transistor, the hearing aid shrunk dramatically. Indeed, the development of transistors in 1948 by Bell Laboratories allowed numerous improvements to be made to the hearing aid, including a dramatic reduction in size. Making use of the transistor and its decreasing dimensions, companies were able to introduce concealable hearing aids. These devices, sometimes referred to as behind-the-ear devices (BTEs), are still available today. Early examples of BTE devices introduced in the 1950's included the Beltone Slimette, the Zenith Diplomat and the Electone 600. With continued advancements in technology, the hearing aid continued to shrink in size to become in-the-ear and in-the-ear-canal devices. Today, some hearing aids are so small that they are implantable.
Conventionally, sound is produced by vibrations such as the movement of a speaker cone, the vibration of a piano string, or the vibration of human vocal cords. These vibrations result in an alternating compression and rarefaction of the air, creating a sound wave that propagates through the air. When produced at wavelengths corresponding to frequencies within the range of human hearing and at sufficient sound pressure levels, these disturbances in the air result in audible sound. The frequency of the resultant sound wave relates to the pitch of the sound, while the amplitude of the sound wave correlates to the loudness of the sound.
FIG. 1 is a block diagram generally representing the features of the mammalian ear. Sounds detected by a human subject reach the ear 101, travel through the external auditory meatus (i.e., the ear canal) 102, to the inner ear 111. The sound wave in the ear canal 102 causes a vibration in the tympanic membrane 103, or ear drum. This vibration is conveyed through the middle ear 104 by way of three small bones commonly referred to as the hammer, anvil and stirrup. The tympanic membrane 103 and the three small bones, or ossicles, carry the sound from the outer ear, through the middle ear to the inner ear. The inner ear includes a spiral-shaped cochlea 111, which is filled with a fluid that vibrates in response to vibrations of the ossicles. Particularly, vibration of the stirrup causes corresponding pressure changes in the fluid of the inner ear. Thus, motion of the stapes is converted into motion of the fluids of the cochlea 111, which some theorize results in a traveling wave moving along the basilar membrane 108.
These pressure changes result in oscillating movements of tiny hair cells, or stereocilia 109, in the inner ear. More particularly, vibrations of the basilar membrane 108 move the bodies of the hair cells 109, deflecting them in a shearing motion, transforming the mechanical energy of sound waves into electrical signals, ultimately leading to an excitation of the auditory nerve. Accordingly, the cochlea 111 converts the mechanical energy of the stapes into electrochemical impulses. These impulses are transmitted via the central auditory nervous system to the auditory processing centers of the brain.
Different sounds are believed to excite different hair cells at different points along what is known as the basilar membrane. The basilar membrane has cross striations, and it varies in width from the base to the apex of the cochlea. Accordingly, different portions of the basilar membrane vibrate at different frequencies. This, in turn, causes different sound frequencies to affect different groupings of the hair cells.
Some audible sound may also reach the inner ear through bone conduction. However, it has been shown that sound conduction through the outer and middle ear is the dominant mechanism for allowing audible sound waves to reach the inner ear and that creating waves with sufficient energy to carrier audio information to the inner ear requires inducement by direct mechanical vibration. Accordingly, sound waves arriving at the listener are predominantly captured by the outer ear and delivered through the hearing system to the inner ear. Sound waves in the range of 20-20,000 Hz are typically only heard through bone conduction when the sound has very high intensity and the listener's ear canals are blocked or audio is otherwise prevented from traveling through the outer and middle ear.